This invention relates to an improved method of making decalcomania (decals) particularly of the heat release type. Heretofore, heat release decals were primarily manufactured by screening or lithography although manufacture feasibility by rotogravure and letterpress was known. The use of offset rotary lithographic presses for printing the varnishes used in making heat release lithographic decals was also known as shown in U.S. Pat. No. 2,640,458. In all of the above mentioned processes the inks and coatings printed contain various organic solvents or oils necessarily included in the printing media to achieve printable viscosities. These inks and coatings are successively printed to achieve heat release decals of the basic layered structure described in U.S. Pat. No. 2,970,076.
The first three layers of a heat release decal are normally not transferable and are comprised of a backing or support material, usually paper, a barrier coat, and a wax release layer. A transferable portion, consisting of design, sealant, overflux, and adhesive layers is deposited thereover. The function of the barrier coat is to prevent absorption of the wax release layer into the paper both at the time of wax application to the paper and transfer of the finished decal to a substrate surface. The release coat is the layer which separates the transferable portion of the decal from the non-transferable portion. The release coat itself is the only portion of a heat release decal which has hitherto been applied by hot melt techniques, such as roll coaters, etc.
Polyethylene glycols having molecular weights in excess of 1000 are commonly employed as the waxes for the release layer as noted in U.S. Pat. Nos. 2,970,076, 3,007,829, and 4,068,033, although both vegetable and mineral waxes can be employed as disclosed in U.S. Pat. No. 2,970,076. The use of polyethylene glycol esters is also known, particularly, if solution coating techniques are to be employed for applying the wax (see for example U.S. Pat. No. 3,533,822).
In a conventional heat releasable decal, it is normal to include an outermost thermoplastic or heat activatable adhesive surface at the opposite or rear side of the design. The term "thermoplastic" as employed relevant to this adhesive layer is not synonymous to the "thermoplastic" term as applied to the thermoplastic inks discussed hereinafter. In both cases the materials reversibly soften with heat, hence the term "thermoplastic". In the present invention when the term is applied to the inks it also implies melt processibility (application), by the methods and apparatus disclosed herein, whereas, the material used for the decal adhesive layer is applied from solution over the design. To avoid the ambiguity of the term thermoplastic, melt processable inks are sometimes referred to as "hot melt" inks or simply "hot color". When, for example, the outermost thermoplastic or heat activatable surface of the decal is pressed against the surface of a preheated vitreous or ceramic article, the heat of the article softens the adhesive surfaces of the decal to a sufficient extent such that the design is adhered to the article being decorated. Concurrently, the heat from the article softens or melts the heat release layer of the backing, thereby causing the backing to release from the design layer. Both actions are accomplished in essentially a single operation in which the decal is pressed against the preheated article. The were with the temporarily adhered vitreous design is thereafter fired in the normal manner to cause the design to become an integral part of the surface of the ware. In the present invention the inks exhibit sufficient pressure sensitivity below their melt points so that the need for a separate adhesion layer is obviated.
The structure and composition of the transferrable portion of the heat release decal depends somewhat on the process used to manufacture the decal. For example screening processes have heretofore proved the most economical process for manufacturing heat release decals. Screening, however, is not without its limitations and drawbacks. The use of solvents in the screening media necessitate drying between the decal layers so that each successive layer can be applied over or adjacent to the previous layers without distortion, smearing, or pick-back of the print. The solvents thereby impart considerable cost to the decal manufacturing process by necessitating both driers and environmental protection controls. Viscosity controls are also required to achieve viscosity stability. Low volatility solvents are often used which require more extensive drying either in terms of time or temperature. Time is the variable usually affected because of the desire not to melt the wax release layer during the drying operation. An increase in drying time, however, means longer driers or slower process speeds. Melting of the wax during drying often results in less acceptable release when transferring the decal to the substrate.
If the backing is paper, it is important to control the dimensions of the paper sheets to ensure proper registration of the subsequent design layers. The dimensions of the paper are directly coupled to its moisture content. Excessive drying time shrinks the sheet by driving out moisture, conversely, increased moisture content resulting from shorter drying time expands the sheet due to absorption. The environmental window in which the paper can be handled is very narrow, thus affecting drying times and temperatures in conjunction with previously mentioned wax problems.
Thermoplastic screening, requiring negligible drying, is not a viable alternative for solving these problems because the heated screens would melt the wax release layer, and thereby, prevent the backing material from accepting the screened prints. This could be prevented by screening overlayers having a melt point lower than the release layer, but this would also be impracticable because the design layer would remelt and smear upon subsequent transfer of the heat release decal to the substrate.
In addition to process limitations, silk-screened decals also have a limitation, in that, they cannot achieve the fine resolution and sharp definition obtainable by lithographic decals; moreover, the thicker layer which results from screening is not always desirable.
In a lithographic decal process, dry color is dusted over the sheet of paper, and adheres only to the printed varnish image. The excess color is then removed from the sheet, leaving the desired image. Due to the aforementioned shortcomings of screened decals, lithographic decals are often used in spite of process disadvantages resulting from the inability of adding colors directly to the lithographic varnish.
The lithographic process is also not without other disadvantages, besides this obvious disadvantage of having to handle, dust, and remove the dry powders after each successive application of varnish. For example, while lithographic decals may have much greater resolution than screened decals, the prints are also much thinner, and as a consequence much higher levels of pigments must be used in the colors. For overglaze decals this necessitates an overflux, i.e., a printed overglaze, which is applied over the colors to improve durability and reduce to safe levels toxic metal release. Although an overflux is sometimes used for screened decals, its use for lithographic decals in food-contact applications is mandatory. Due to the tendency of the dry colors to stick to the overflux layer, a sealant layer is required in the construction of a lithographic decal between the design layer and the overflux layer as shown in U.S. Pat. No. 4,068,033. In general, lithographic heat release decals have been found more difficult to manufacture than screened decals, such that, their use has been limited.